1. Field of the Invention
Warfarin; microcrystalline warfarin; anticoagulants; rodenticides.
2. Prior Art
The compound 3-(alpha-acetonylbenzyl)-4-hydroxy-coumarin (warfarin) is well-established and widely-employed as an oral anticoagulant and rodenticide. The rodenticide effect of the compound is due to the anticoagulant and blood thinning effect of the compound acting as an anticoagulant.
The compound 3-(alpha-acetonylbenzyl)-4-hydroxy-coumarin exists in both enol (acid) and keto configurations. The enol form is generically known as warfarin acid and is adopted by the U.S.P. XVIII as the reference standard for purity, being considered the active compound, despite its normal use in the form of its sodium salt. This form will hereinafter be referred to as warfarin acid, enol warfarin, or simply warfarin.
The sodium salt of 3-(alpha-acetonylbenzyl)-4-hydroxycoumarin, as described in the U.S. Pharmacopeia in its most recent revision (XVIII), under the generic name of Sodium Warfarin, page 674, shows inconsistency in its chemical composition due to its recrystallization from isopropyl alcohol in the form of a clathrate containing varying amounts of warfarin, sodium, water, and isopropyl alcohol, despite the great effort which has been exerted to obtain a pure cyrstalline compound of a definite chemical structure (see U.S. Pat. Nos. 3,077,481 and 3,246,013). These efforts were exerted because of the belief that solubility in water of the sodium salt of 3-(alpha-acetonylbenzyl)-4-hydroxy-coumarin was a requirement for absorption from the gastrointestinal tract (see U.S. Pat. No. 2,777,859). It was later shown that solubility of the sodium salt in water is not the important factor in absorption of 3-(alpha-acetonyl-benzyl)- 4-hydroxycoumarin from the alimentary canal, but that the sodium salt is only a convenient chemical means of providing the enol or acid form of the anticoagulant 3-(alphaacetonylbenzyl)- 4-hydroxycoumarin, which is the active anticoagulant and the compound actually absorbed from the gastrointestinal tract (Robert A. O'Reilly, Annals of the New York Academy of Sciences, Vol. 226, pp. 293-308 [Nov. 26, 1973]). This fact has justified the elaborate work which has been undertaken to obtain sodium warfarin in a pure crystalline form. In other words, it was confirmed that the hydrophobic enol or acid form is more readily and rapidly absorbed than the hydrophilic sodium salt. What really happens, then, in the alimentary canal, is that the sodium salt is converted to the enol or acid form, which is the form actually absorbed and responsible for the anticoagulant action. Confirming this finding, the dissolution rate test for sodium warfarin tablets as published by O'Reilly, R. A. in Bioavailability of Drugs (1972 ), requires that the pharmaceutical form be exposed for thirty (30) minutes to 0.1 N hydrochloric acid, which is intended to precipitate the warfarin acid, prior to conducting the dissolution rate test at a physiological pH of 7.4. However, the warfarin acid of commerce is amorphous or large crystals and even the U.S.P. Reference Standard crystals, recrystallized from ethanol, are more than 2,000 microns in length and average 100 microns in diameter, being in the form of large needles. [This is in direct contrast to the warfarin acid crystals of the present invention which are microcrystalline in nature, not greater than 4 microns in length, ordinarily 0.1 to 4 microns in length, having an average length of 0.62 micron (average size of 20 crystals), are translucent, have sharp edges and are also needle shaped. The term "microcrystalline", by definition, is applied to crystals which in length and diameter do not exceed 10 microns. Alfonso, A. S. et. al., Journal of Pharmaceutical Sciences 60, No. 10, pp. 1572-1574 (Oct. 1971).]
It has also been confirmed that drugs which are insoluble in water, when administered orally in a microcrystalline form in crystal sizes below 10 microns, attain higher serum levels more rapidly than the same drugs administered in larger crystal or particle size. (Alfonso, A. S. and Nailz, V. R., Journal of Pharmaceutical Sciences 60, No. 10, pp. 1572-1574 [Oct. 1971]).
The warfarin acid of commerce is amorphous and is obtained chemically pure only by crystallization from ethyl alcohol. Its method of manufacture is disclosed in U.S. Pat. No. 2,752,360, which describes its purification by crystallization from hot ethyl alcohol. This warfarin acid of commerce can be used as a rodenticide, but it is not 100% pure and is known chemically to be a mixture having unknown proportions of the keto and enol warfarin. It cannot be used clinically, because only the pure enol form is desirable and acceptable by the medical profession for its anticoagulant action. The impurities inherent in the commercially available warfarin acid of commerce, moreover, cannot be removed by recrystallization from alcohol or other organic solvents. Pure warfarin from pharmaceutical and clinical use can be obtained from the commercially available product only if it is converted completely to its enol or acid form through a process which would also remove all phenolic impurities. Up to the present time, only by use and employment of the sodium salt form thereof, i.e., "Crystalline Sodium Warfarin, USP", has this been possible. For example, in the manufacture of the warfarin sodium salt (see U.S. Pat. Nos. 2,777,859, 2,765,321, and 3,077,481), the initial step calls for the use of sodium hydroxide and an excess of warfarin which acts as a buffer to avoid degradation of the warfarin molecule by the strongly alkaline sodium hydroxide. The excess of warfarin is then recovered and reused in a relatively uneconomic but continuous process of converting the warfarin to its sodium enolic salt. It would be highly desirable to have available pure warfarin acid in a different and definite crystalline form and by a process which obviated the previously unavoidable disadvantages already mentioned.
It is accordingly one object of this invention to provide directly for oral use the enol or acid form of warfarin in a pure pharmaceutical grade and of a microcrystalline structure which has hitherto not been available, and by a novel process which has likewise been hitherto unavailable.